Tropical forests are home to a rich diversity of tree species. Scientists have previously argued competition among "natural enemies" explains the ecosystem's unique biodiversity. But new research suggests such an explanation is not sufficient on its own.

The Janzen-Connell hypothesis posits that each species in a tropical forest has a natural enemy. Competition between natural enemies, theoretically, keeps species from dominating the ecosystem, creating the space and stability needed for biodiversity.

When ecologists at Yale University tested the theory using statistical modeling and biodiversity data collected on Barro Colorado Island in Panama, they found some species are more susceptible to natural enemies than others. The simulations showed competition among natural enemies doesn't always promote stability and biodiversity.

"Our analysis suggests that the natural enemies on this island may actually be harming the ability of many species to survive rather than uniformly helping species co-exist," Simon Stump, a postdoctoral associate at the Yale School of Forestry and Environmental Studies, said in a news release. "This finding calls into question -- or at least adds a significant caveat -- to a major hypothesis in community ecology. At the very least it suggests that other explanations may be needed."

According to the Janzen-Connell hypothesis, the evolutionary success of each tree species is kept in check by herbivorous insects, fungal pathogens or some other type of specialized enemy, which depress the survival rates of the trees' seeds and seedlings.

The seeds and seedlings near their source are most likely to be preyed upon by these natural enemies, a phenomenon called conspecific negative density dependence. The phenomenon creates space for the success of rarer species.

Critics of the Janzen-Connell hypothesis contend that some common tree species are more susceptible to natural enemies than others. Critics also argue some rare species are not immune to the enemies of other species.

Yale ecologists designed a model to test the Janzen-Connell hypothesis in a forest with varying levels of vulnerability to natural enemies among tree species. The simulations showed the variability can have a destabilizing effect, curbing biodiversity.

The findings -- published this week in the journal Ecology Letters -- support the conclusions of previous surveys of Barro Colorado Island, which showed rare tree species on the island are actually more susceptible to natural enemies than common species.

"Thus, contrary to the commonly held assumption that conspecific negative density dependence always promotes tree diversity, our new theoretical study suggests that it may actually reduce the number of species that coexist," said Liza Comita, an assistant professor of tropical forest ecology at Yale. "Our study shows how combining data from field with theoretical models can result in big advances in our understanding of complex systems, such as tropical forests."

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